Climate change information is increasingly available and robust at regional scale for impacts and risk assessment. Climate services and vulnerability, impacts, and adaptation studies require regional scale multi decadal climate observations and projections. Since AR5, the increased availability of coordinated ensemble regional climate model projections and improvements in the level of sophistication and resolution of global and regional climate models, completed by attribution and sectoral vulnerability studies, have enabled the investigation of past and future evolution of a range of climatic quantities that are relevant to socio-economic sectors and natural systems. Chapter 12 consolidates core physical knowledge from preceding AR6 WGI chapters and post-AR5 climate impact assessment literature to assess the spatio-temporal evolution of the climatic conditions that may lead to regional scale impacts and risks (following the sectoral classes adopted by AR6 WGII) in the world’s regions (presented in Chapter 1)
The Climatic Impact-Driver (CID) framework adopted in Chapter 12 allows for assessment of changing climate conditions that are relevant for sectoral impacts and risks assessment. CIDs are physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems and are thus a priority for climate information provision. Depending on system tolerance, CIDs and their changes can be detrimental, beneficial, neutral, or a mixture of each across interacting system elements, regions and society sectors. Each sector is affected by multiple CIDs, and each CID affects multiple sectors. A CID can be measured by indices to represent related tolerance thresholds.
The current climate in most regions is already different from the climate of the early or mid 20th century with respect to several CIDs. Climate change has already altered CID profiles and resulted in shifts in the magnitude, frequency, duration, seasonality, and spatial extent of associated indices. Changes in temperature-related CIDs such as mean temperatures, growing season length, extreme heat, frost, have already occurred and many of these changes have been attributed to human activities. Mean temperatures and heat extremes have emerged above natural variability in all land regions with high confidence. In tropical regions, recent past temperature distributions have already shifted to a range different to that of the early 20th century. Ocean acidification and deoxygenation have already emerged over most of the global open ocean, as has reduction in Arctic sea ice. Using CID index distributions and event probabilities accurately in both current and future risk assessments requires taking into account the climate change–induced shifts in distributions that have already occurred.
Several impact-relevant changes have not yet emerged from the natural variability, but will emerge sooner or later in this century depending on the emission scenario. Increasing precipitation is projected to emerge before the middle of the century in the high latitudes of the Northern hemisphere. Decreasing precipitation will emerge in a very few regions (Mediterranean, South Africa, South Western Australia) by the mid-century. The anthropogenic forced signal in near-coast relative sea-level rise will emerge by mid-century RCP8.5 in all regions with coasts, except in the West Antarctic region where emergence is projected to occur before 2100. The signal of ocean acidification in the surface ocean is projected to emerge before 2050 in every ocean basin. However, there is low evidence of emerging drought trends above natural variability in the 21st century.
Every region of the world will experience concurrent changes in multiple CIDs by mid-century challenging the resilience and adaptation capacity of the region. Heat, cold, snow and ice, coastal oceanic, and CO2 at surface CID changes are projected with high confidence in most regions, indicating worldwide challenges, while additional region-specific changes are projected in other CIDs that may lead to more regional challenges. High confidence increases in some of the drought, aridity, and fire weather CIDs will challenge, for example, agriculture, forestry, water systems, health and ecosystems in Southern Africa, the Mediterranean, North Central America, Western North America, the Amazon regions, South-western South America, and Australia. High confidence changes in snow, ice and pluvial or river flooding changes will pose challenges for, for example, energy production, river transportation, ecosystems, infrastructure and winter tourism in North America, Arctic regions, Andes regions, Europe, Siberia, Central, South and East Asia, Southern Australia and New Zealand. Only a few CIDs are projected to change with high confidence in the Sahara, Madagascar, Arabian Peninsula, Western Africa, Small Islands; however, the lower confidence levels for CID changes in these regions can originate from knowledge gaps or model uncertainties, and does not necessarily mean that these regions have relatively low risk.
Worldwide changes in heat, cold, snow and ice, coastal, oceanic and CO2-related CIDs will continue over the 21st century, albeit with regionally varying rates of change, regardless of the climate scenario. In all regions, there is high confidence that, by 2050, mean temperature and heat extremes will increase, and there is high confidence that sea surface temperature will increase in all oceanic regions, excepting the North Atlantic. With the exception of a few regions with substantial land uplift, relative sea level rise is very likely to virtually certain (depending on the region) to continue along the 21st century, contributing to increased coastal flooding in most low-lying coastal areas and coastal erosion along most sandy coasts, while ocean acidification is virtually certain to increase. It is virtually certain that atmospheric CO2 at the surface will increase in all emissions scenarios until net zero emissions are achieved. Glaciers will continue to shrink and permafrost to thaw in all regions where they are present. These changes will lead to climate states with no recent analogue that are of particular importance for specific regions such as tropical forests or biodiversity hotspots.
A wide range of region-specific CID changes relative to recent past are expected with high or medium confidence, by 2050 and beyond. Most of these changes are concerning CIDs related to the water cycle and storms. Agricultural and ecological drought changes are generally of higher confidence than hydrological drought changes, with increases projected in North and Southern Africa, Madagascar, Southern and Eastern Australia, some regions of Central and South America, Mediterranean Europe, Western North America and North Central America. Fire weather conditions will increase by 2050 under RCP4.5 or above in several regions in Africa, Australia, several regions of South America, Mediterranean Europe, and North America . Extreme precipitation and pluvial flooding will increase in many regions around the world. Increases in river flooding are also expected in Western and Central Europe and in polar regions, most of Asia, Australasia and North America, South American Monsoon and Southeastern South America. Mean winds are projected to slightly decrease by 2050 over much of Europe, Asia, and Western North America, and increase in many parts of South America except Patagonia, West and South Africa and Eastern Mediterranean. Storms are expected to have a decreasing frequency but increasing intensity over the Mediterranean, most of North America, and increase over most of Europe. Enhanced convective conditions are expected in North America. Tropical cyclones are expected to increase in intensity despite a decrease in frequency in most tropical regions. Climate change will modify multiple CIDs for small islands in all ocean basins, most notably those related to heat, aridity and droughts, tropical cyclones and coastal impacts.
The level of confidence in the projected direction of change in CIDs and the intensity of the signal depend on mitigation efforts over the 21st century, as reflected by the differences between end-century projections for different climate scenarios. Dangerous humid heat thresholds, such as the NOAA HI of 41°C, will be exceeded much more frequently under SSP5-8.5 scenario than under SSP1-2.6 and will affect many regions. In many tropical regions, the number of days per year where a HI of 41°C is exceeded will increase by more than 100 days relative to the recent past under SSP5-8.5, while this increase will be limited to less than 50 days under SSP1-2.6. The number of days per year where temperature exceeds 35°C will increase by more than 150 days in many tropical areas, such as the Amazon basin and South-east Asia under SSP5-8.5, while it is expected to increase by less than 2 months in these areas under SSP1-2.6 (except for the Amazon Basin). There is high confidence that the total length of sandy shorelines around the world that are projected to retreat by more than 100 m will be 35% greater under RCP8.5 (~130,000 km) compared to RCP4.5 (~ 95,000 km) by the end of the century. The frequency of the present-day 1-in-100-yr extreme sea level (represented here by extreme total water level) event, in a globally averaged sense, is projected to become an event that occurs multiple times per year under RCP8.5, while under RCP 4.5 it is projected to become a 1-in-5-yr event, representing a 5 fold difference between the two RCPs).
There is low confidence in past and future changes of several CIDs. In nearly all regions there is low confidence in changes in hail, ice storms, severe storms, dust storms, heavy snowfall, and avalanches, although this does not indicate that these CIDs will not be affected by climate change. For such CIDs, observations are short-term or lack homogeneity, and models often do not have sufficient resolution or accurate parametrizations to adequately simulate them over climate change time scales.
Many global- and regional-scale CIDs have a direct relation to global warming levels (GWLs) and can thus inform the hazard component of ‘Representative Key Risks’ and ‘Reasons for Concern’ assessed by AR6 WGII. These include heat, cold, wet and dry hazards, both mean and extremes; cryospheric hazards (snow cover, ice extent, permafrost) and oceanic hazards (marine heatwaves). For some of these, a quantitative relation can be drawn. For example, with each degree of GSAT warming, the magnitude and intensity of many heat extremes show a linear change, while some changes in frequency of threshold exceedances are exponential; arctic temperatures warm about twice as fast as GSAT; global SSTs increase by ~80% of GSAT change; Northern Hemisphere spring snow cover decreases by ~8% per 1°C. For other hazards (e.g., ice sheet behaviour, glacier mass loss, global mean sea-level rise, coastal floods and coastal erosion) the time and/or scenario dimensions remain critical and a simple relation with GWLs cannot be drawn, but still quantitative estimates assuming specific time frames, and/or stabilized GWLs can be derived. Model uncertainty challenges the link between specific GWLs and tipping points and irreversible behavior, but their occurrence cannot be excluded and their chances increase with warming levels.
Since AR5, climate change information produced in climate service contexts has increased significantly due to scientific and technological advancements and growing user demand. Climate services involve the provision of climate information in such a way as to assist decision-making. These services include appropriate engagement from users and providers, are based on scientifically credible information and expertise, have an effective access mechanism, and respond to user needs. Climate services are being developed across regions, sectors, timescales and target users.
Climate services are growing rapidly and are highly diverse in their practices and products. The decision-making context, level of user engagement and co-production between scientists, practitioners and intended users are important determinants of the type of climate service developed and its utility supporting adaptation, mitigation and risk management decisions. User needs and decision-making contexts are very diverse and there is no universal approach to climate services.
Realization of the full potential of climate services is often hindered by limited resources for the co design and co-production process, including sustained engagement between scientists, service providers and users. Further challenges relate to climate services development, provision of climate services, generation of climate service products, communication with users, and evaluation of the quality and socio-economic value of climate services. The development of climate services often uncovers and presents new research challenges to the scientific community.